In waveguide QED, atoms are commonly regarded as point-like dipoles because their sizes are in general much smaller than the wavelengths of the waveguide modes they interact with. Recent experiments show that such an approximation is no longer valid when (artificial) atoms interact with a surface acoustic wave whose wavelength can be even much smaller than microwave photons [1]}, [2]}, [3]}. Moreover, it is also possible to couple a single atom with bent waveguides at two or more points separated by distances much larger than one wavelength. Such configurations are referred to as giant atoms [4]}, which can exhibit striking effects such as frequency-dependent decays and Lamb shifts [4]}, [6]}, chiral emission [7]}, [8]}, and oscillating bound states [9]}. Recently, decoherence-free interactions between braided giant atoms are theoretically proposed [10]} and experimentally verified [11]}, where the giant atoms are immune to emitting photons to the waveguide yet they still interact effectively with each other. Moreover, giant-atom structures have also been extended to two or higher dimensions with optical lattices of cold atoms [12]}. These seminal works provide new inspirations for many applications in quantum simulating and quantum computation.
